Part 15 (2/2)

THE MICROSCOPE, THE TELESCOPE, AND THE MAGIC-LANTERN

The simple microscope--Use of the simple microscope in the telescope--The terrestrial telescope--The Galilean telescope--The pris telescope--The parabolic ic-lantern--The bioscope--The planeat a vase, three inches high, situated at A, a foot away If ere to place another vase, B, six inches high, at a distance of two feet; or C, nine inches high, at three feet; or D, a foot high, at four feet, the ie on the retina would in every case be of the same size as that cast by A We can therefore lay down the rule that _the apparent size of an object depends on the angle that it subtends at the eye_

[Illustration: FIG 119]

To see a thing o nearer to it; and if it be very small, we hold it close to the eye There is, however, a lie The normal eye is unable to adapt its focus to an object less than about ten inches away, termed the ”least distance of distinct vision”

THE SIMPLE MICROSCOPE

[Illustration: FIG 120]

A lass comes in useful ant to exalass is a lens of short focus, held at a distance so 120), from the object The rays from the head and tip of the pin which enter the eye are denoted by continuous lines As they are deflected by the glass the eye gets the _ier pin is situated a considerable distance behind the real object in the plane in which the refracted rays would meet if produced backwards (shown by the dotted lines) The effect of the glass, practically, is to remove it (the object) to beyond the least distance of distinct vision, and at the sale it subtends at the eye, or, what ae formed on the retina[22] It follows, therefore, that if a lens be of such short focus that it allows us to see an object clearly at a distance of two inches--that is, one-fifth of the least distance of distinct vision--we shall get an ier in diameter than would be possible without the lens

The two sinified should be nearer to the lens than the principal focus, F We have already seen (Fig 109) that rays coe as a parallel pencil These the eye can bring to a focus, because it nor to the power of ”acco 121), the eye lens thickening the necessary alass a bit nearer than F to get full advantage of proximity If we had the object _outside_ the principal focus, as in Fig 122, the rays froathered to a sharp point by the eye lens, as it cannot _flatten_parallel rays

[Illustration: FIG 121]

[Illustration: FIG 122]

USE OF THE SIMPLE MICROSCOPE IN THE TELESCOPE

[Illustration: FIG 123]

Let us now turn to Fig 123 At A is a distant object, say, a hundred yards away B is a double convex lens, which has a focal length of twenty inches We e of the object is cast by the lens at C If the eye were placed at C, it would distinguish nothing But if withdrawn to D, the least distance of distinct vision,[23] behind C, the ie really is at C is proved by letting down the focussing screen, which at once catches it Now, as the focus of the lens is twice _d_, the ie as the object would appear if viewed directly without the lens We nification = focal length of lens -------------------- _d_

[Illustration: FIG 124]

In Fig 124 we have interposed between the eye and the object a slass of 2-1/2-inch focus, so that the eye can now clearly see the inifies the inifies it four tinification is 2 4 = 8 ti the focus of B (which corresponds to the object-glass of a telescope) by the focus of the eye-piece, thus:--

20 ____ = 8 2-1/2

The ordinary astronolass at one end of the tube, and a very short focus eye-piece at the other

To see an object clearly one merely has to push in or pull out the eye-piece until its focus exactly corresponds with that of the object-glass

THE TERRESTRIAL TELESCOPE

An astronoes This inversion is inconvenient for other purposes So the terrestrial telescope (such as is commonly used by sailors) has an eye-piece conify the i eye-piece